The invention relates to a device for alternating examination of a measurement object by means of MPI (=“Magnetic Particle Imaging”) and MRI (=“Magnetic Resonance Imaging”) within a magnetic system comprised of at least two magnetic field generating elements, wherein the magnetic system has a first examination region for the MRI operation, in which a homogenous magnetic field is generated, and wherein the magnetic system has a second examination region for the MPI operation, in which a spatially strongly varying magnetic field profile is generated, the field vectors thereof being different at all spatial points with regard to direction and/or amount and comprising one spatial point of zero field magnitude.
Such a device is known from reference [3] (Weizenecker et al., 2009).
In recent decades, several tomographic imaging methods have been invented, including, for example, computer tomography (CT) by Hounsfield in 1969, magnetic resonance tomography (MRI) by Lauterbur and Mansfield in 1973, or positron emission tomography (PET) by Ter-Pogossian and Phelps in 1975. With the constant development of hardware, sequence and/or reconstruction algorithms, imaging methods are assuming an ever more important role in medical diagnosis today. By combining individual imaging methods into so-called hybrid systems (for example, PET-CT in clinical use since 2001 and MRI-PET in clinical use since 2010), it has been possible to further improve the diagnostic accuracy of imaging methods. A common feature of all hybrid systems is that complementary information from the individual modalities is combined synergistically and/or overlaid graphically. So, for example, CT data from a PET-CT hybrid system are used for morphological information and for attenuation correction of the PET data.
In 2001, a further tomographic imaging method called magnetic particle imaging (MPI) was invented by Gleich and Weizenäcker (DE10151778A1). This recent and fast developing volumetric imaging method is used to detect the spatial distribution of applied superparamagnetic nanoparticles (SPIO). This method offers spatial as well as high temporal resolution performance (see references [1-3]).
The fundamental principle of MPI is based on the excitation of nanoparticles using a magnetic field that changes over time, the so-called “drive field” (DF), with an excitation frequency f0. The non-linear magnetization curve of the SPIOs produce harmonics of F0 as the particle response, which are detected by means of receiver coils and are used for image reconstruction. Because tissue produces a negligibly small non-linear response to the excitation frequency f0, this method offers a high contrast by acquisition of the particle response only. Spatial encoding is based on the effect that the particle magnetization becomes saturated above a certain magnetic field strength. As a result of magnetic excitation at frequency f0, the magnetization of the saturated SPIOs only changes minimally and therefore its contribution to the particle response is non existent or minimal. In order to exploit this saturation effect, a static magnetic field gradient—the so-called “selection field” (SF)—with a field-free point (FFP) is generated. Starting out from the FFP, the magnetic field strength increases in all spatial directions.
Such a magnetic field profile can be produced, for example, with permanent magnets with opposed magnetization directions or by using a Maxwell electromagnetic coil pair. Only particles in the direct vicinity of the FFP are excited by the saturation effect and thus contribute to the response of the particles. The extent of the FFP and thus the sensitivity of the MPI method is dependent on the magnetic field strength with which the particles reach saturation, and on the gradient strength of the SF, with which the magnetic field increases starting from the FFP (see references [4, 5]). To allow volumetric imaging, the FFP is controlled relative to the object under examination by, for example, the superposition of additional magnetic fields and/or the mechanical movement of the object under examination.
The MPI quantitative method with its high level of sensitivity and high temporal resolution offers promising non-invasive application possibilities in the field of molecular and medical imaging such as, for example, cell tracking or cancer diagnosis and in the field of cardiovascular diagnosis and blood vessel imaging. Unlike other imaging methods such as, for example, CT and MRI, MPI image data sets thus acquired still produce a relatively low spatial resolution in the millimeter range. This limited resolution is due to the currently available nanoparticles and the magnetic field gradients that are technically feasible. Moreover, data having high sensitivity exclusively with respect to the applied nanoparticles can provide information about the quantitative distribution of these nanoparticles, although this contains little morphological information. This makes it extremely difficult to uniquely assign the measured distribution of the particles to their morphological location of origin.
Other volumetric imaging methods such as, for example, the MRI method, which has been in clinical use for a long time, are very suitable for acquiring high-resolution morphological information. The MRI technique is based on a highly homogeneous magnetic field and electromagnetic alternating fields in the radio-frequency range, with which certain atomic nuclei in the object under examination can be excited (see reference [6]). The excited atomic nuclei themselves emit alternating electromagnetic fields, which induce electric signals in the receiver coil. By using multiple magnetic field gradients, the signal is spatially encoded and can be reconstructed by suitable algorithms. MRI not only enables the acquisition of high-resolution anatomical information with varied soft tissue contrasts in the sub-millimeter range, but also offers further differentiated techniques, which provide access to many physiological parameters, such as water diffusion or permeability. If MR spectroscopic imaging is used, metabolic and biochemical processes can also be represented spatially. Unlike MPI, the MRI technique is a relatively insensitive and slow imaging method with acquisition times in the seconds to minutes range.
The unique characteristics of both volumetric imaging modalities make MPI and MRI largely complementary in terms of the information they contain. Superior diagnostic validity can be achieved by combining both methods and making synergetic use of their properties—the high sensitivity and temporal resolution of the MPI technique and the varied soft tissue contrasts and therefore excellent morphological information of the MRI technique. Superposition/fusion of both complementary image data sets has so far only been implemented using two separate or independent modalities of MPI and MRI (see reference [3]) as no integrated combined device (hybrid device) of these two modalities is yet available anywhere in the world.
However, using two separate modalities poses many difficulties. These include the co-registration of both data sets with different reference coordinates, which is hampered by the largely unavoidable shifting and deformation caused by repositioning and/or transportation of the object under examination from one modality to the other. Intermodality transportation also reduces the direct correlation between the two data sets in time. Further logistical problems result, for example, in studies with small animals, which require the laboratory animal to be continuously anesthetized. The provision of two standalone modalities also means high costs and space requirements. A combination of both modalities in an integrated hybrid device was disclosed in JP-2009195614-A2, although the hybrid device described therein has two different modes (MRI and MPI), which are implemented by switching partial coil systems on and off.
The object of this invention is to improve a device having the characteristics defined above in a low-cost manner and by using simple technical means in such a way that the difficulties described above in combining the two modalities into an integrated hybrid device are reduced or avoided, wherein the total space requirement for both modalities is reduced and the complexity of the integrally designed hybrid system is minimized and switching between the two modes with respect to the magnetic field profile is no longer necessary.